Petrogenesis of Late Miocene high Ba-Sr granitoids in eastern Pamir, northwest Tibetan Plateau: Insights into lithospheric mantle evolution and geodynamic processes during India-Asia bidirectional subduction

Abstract Cenozoic crustal thickening and surface uplift in Pamir, northwest Tibetan Plateau is controlled by India-Asia continental convergence and post-collisional subduction processes. However, the nature and evolution of post-collisional subducted lithosphere and the associated deep dynamic processes remain unclear. In this study, we report new geochemistry, mineral chemistry and geochronology for three plutons (Kuzigan, Karibasheng and Zankan) in eastern Central Pamir, to constrain their petrogenesis and help understand the associated post-collisional geodynamic processes. LA-ICP-MS U-Pb zircon dating indicates that the Kuzigan and Karibasheng plutons were emplaced in the Late Miocene (ca. 11.2–10.7 Ma). Whole-rock compositions are characterized by high Ba (1890–7550 ppm) and Sr (1050–3570 ppm) as well as crust-like Sr-Nd-Pb-Hf-O isotopic compositions, thus with a marked affinity to high Ba-Sr granitoids. Mafic to intermediate syenites have moderate Mg# values (up to 55) as well as Cr (up to 104 ppm) and Ni (up to 59 ppm) contents, indicative of a mantle source. They have negative εNd(t) (-9.22 to -8.87) and εHf(t) (-11.8 to -6.49), combined with high (87Sr/86Sr)i (0.7099–0.7109) and δ18Ozrn (+9.99‰ to +10.9‰), as well as enrichment in large ion lithophile elements (LILEs, e.g. Ba, U, Th and K) and depletion in high field strength elements (HFSEs, e.g. Nb, Ta, P and Ti). These features suggest an origin from enriched lithospheric mantle, modified by subduction-related melts. Sr-Nd-Pb isotope modelling indicates contributions from both the Indian plate (~20–30%) and the Asian plate (~1–3%). Associated syenogranites exhibit a mineral assemblage and isotopic compositions similar to the syenites, as well as parallel trace-element patterns, indicating a common magma source. Their geochemical variability likely reflects fractional crystallization of clinopyroxene, biotite, rutile, feldspars and accessory phases (titanite, zircon, apatite and allanite). The Karibasheng monzogranites, by contrast, have uniformly high SiO2 (70.9–72.5 wt.%) but lower MgO (0.36–0.48 wt.%) compared to the syenitic rocks. Their low εNd(t) (-7.46 to -6.88) and εHf(t) (-11.9 to -5.80), along with high (87Sr/86Sr)i (0.7091–0.7092) and δ18Ozrn (+8.75‰ to +10.7‰), point to derivation from the remelting of ancient metasedimentary rocks. Combining these data with regional geochronology and previous geophysical studies, we propose a west-to-east magmatic migration in the Central Pamir and a gradual delamination model to explain the origin of Miocene magmas. Blocked by the subducting Indian plate, continental crust foundered resulting in asthenosphere upwelling and subsequent melting of the lithosphere, producing high Ba-Sr syenites. Given the spatial-temporal distribution of Pamir magmatism and the associated regional geology, we suggest that the deep geodynamic evolution of the lithosphere was the primary driver of Late Cenozoic tectonic uplift in the Pamir. This study highlights the deep link between continental delamination, mantle processes and generation of Miocene magmas in Central Pamir and provides new insights into episodic uplift of Pamir.